Direct-current amplifier



Dec. 7, 1948. B. L .WELLER DIRECT CURRENT AMPLIFIER Filed Feb. 27, 1945 i Ztftf'rzess es mz( ya Patented Dec. 7, 1948 DIRECT-CURRENT AMPLIFIER.

aman L. Weiler, niemand, wasn., signor u the United States of America as represented by the United States Atomic Energy Commission Application February 27, 1945, Serial No. 580,045 4 Claims. (Cl. 179-171) My invention relatesl to an electrical amplifying system and method suitable for the amplification of direct currents and relatively low frequency alternating currents. More specifically, my invention relates to an electrical system and method of coupling a discharge device to an amplifier Vso as to reduce or entirely eliminate the effect of cath'ode drift of the discharge device on the output of the amplifier. The system may be regarded broadly as a `current; amplifier and voltage regulator.

In the past, discharge devices, such as electrometer tubes of various types, have been used for amplifying very minute electrical currents. A

` troublesome effect, -commonly known as cathode drift, was generally encountered, especially in highly sensitive discharge devices, manifesting itself in a slow erratic fluctuation of output current or voltage and being detected readily when the direct current voltage applied to the input for amplification is maintained at a constant value or zero. It is generally believed that cathode drift results from changes in fllamentcurrent or from changes in the active filament coating or minute changes in the cathode temperature; in any event, cathode drift manifests itself in changes of electron emission currents emitted by the cathode. Cathode drift is especially important and troublesome when it appears in or near the input stage of a multi-stage amplifier because although minute, its effect is magnified substantially by the fact that that very high current amplification is obtained, for example an amplification of the order of 250,000, in certain electrometer tubes for amplifying very small currents. Obviously, if additional amplifying stages are added, the effect of cathode drift is still further amplified thus introducing considerable error in the output signal.

On object of my invention is to eliminate or substantially reduce the effect of cathode drift in an electrical amplifier.

4A more specific object of my invention is to provide an electrical system for coupling a pair of space charge devices in a manner so that cathode-drift in one of the'devices will not be amplified and4 translated through the otherdethe faithful amplification of direct current, or

alternating current at low frequencies, particularly those below 1,000 cycles per second.

A further and more specific object f my invention is to provide a voltage regulating system in a high-sensitivity, multi-stage amplifier, `that is useful to eliminate or substantially reduce the effect of cathode drift originating, for example, in a highly sensitive electrometer tube used in th'e first stage 'of such amplifier.

' A still further and more specific object of my invention is to provide a voltage regulating system providing highly amplified corrective effects to compensate for cathode drift.

Another object of my invention is to provide a novel method of translating amplified currents so as to render them devoid of distortion caused by cathode drift.

Other objects and advantages will become more apparent from a study of the following specification taken together with the drawing wherein a schematic diagram shows an illustrative direct current electrical amplifying system embodying the teachings of my invention.

Referring more particularly to the drawing, T1 denotes an electric discharge device or tube and is illustrated as a high vacuum, space charge .tetrode tube ofA high sensitivity, comprising an input or control electrode or grid i, an anode or plate 2, a space charge grid or electrode 3, and a thermionic cathode 4. Tube T1 is, preferably, one having a high input resistance and a very low grid current so as to give maximum benefits when used to measure or translate very small currents or voltages. An example of such' tube is the G. E. Pliotron tube type FP-54, having an input resistance `of 101s ohms, a current amplification of 250,000 when used with an input resistor of 1012 ohms, and a voltage amplification factor of unity. Such tube is particularly adapted for the amplification or measurement of currents of less than 10-9 amperes. A voltage source, indicated by battery 5, provides the plate voltage, the polarity of its terminals being indicated by symbols and The anode 2 and cathode I of tube T1 are energized by the plate voltage source through a series connected plate resistor 6. The inner grid, that is, the space charge grid 3, is connected to the positive terminal 1 of the plate voltage source 'through resistor 8 and series resistor 9 so that the space charge grid 3 will be maintained at a positivel potential with respect to cathode I. A voltage source or grid bias battery l0 is connected between the negative terminal Il of the plate voltage source and the cathode 4 to give a negative bias to the input or control electrode I, for` example, in the neighborhood of -2 to -4 volts with respect to cathode I. Suitable operation is obtained if anode 2 is mainpressed on control grid I5.

resistor 8 is to increase the positive bias on grid 3 tained at +6 volts with respect to cathode l, space charge grid 3 at +4 volts with respect to cathode I, the battery 5 having apotential of 90 volts. The input signal is applied between control grid I and the negative terminal II ofthe plate voltage source 5.

An electric discharge device or tube T2 is provided for amplifying and regulating the output current of tube T1, and is illustrated as a double triode having a common cathode I2, two anodes I3 and I4, and two control electrodes or grids I5 and I6, associated with anodes I3 and I4, respectively Tube T2 preferably has a high amplification factor, and tubes such as. for example, the RCA type 6SC7 or 6SL7 are suitable. As illustrated, the triode at the left of tube T2 comprises the amplifier section, and the triode at the right, the regulating section of the double triode tube. The grid I5 of the amplifier section is connected to the plate 2 of the tetrode tube T1 and the common cathode I2 is connected to the negative terminal II of the plate supply through a cathode resistor I 1. The anode I3 of the amplifier section is connected to the positive terminal of the plate supply through a series load resistor I8. It is the voltage drop across resistor I8 that is to be regulated or controlled since such voltage drop is fed into the output or subsequent stages of the amplifier. In the control or regulating section of the double triode tube T2, control grid I5 is connected to the space charge grid 3 of tetrode tube T1 through the resistor Il, and the anode I4 is connected directly to the positive terminal 'I of the plate voltage supply. Terminals II and I8 constitute the output terminals of tube T2.

The above-described amplifying system is useful to amplify and transmit a signal fed to the control grid I of the space charge tetrode T1. If it is assumed that the incoming signal voltage is such as to cause the control grid I to become more positive with respect to the cathode 4, such increase in positive signal voltage increases the conductivity of tube T1 and results in an increase in the voltage drop across plate resistor 6. Anode 2 thus becomes more negative thankunder static conditions with respect to the potential of terminal II as does also the control grid I5 of the amplifier section of tube T2.

The increase in positive signal voltage will cause la greater number of the electrons emitted from the cathode 4 to be accelerated through the inner or space charge grid 3 resulting in a decrease in the space charge grid current, and in turn resulting in a decrease in the voltage drop across the resistors 8 and 9 connected in series therewith. This decrease in voltage drop results in an increase in the positive potential of the space charge grid 3 as well as an increase in positive potential of the control grid I6 of the regulator section of double triode tube T2 connected therewith.

This increase in positive potential of control grid I5 results in an increase in plate current in the regulator section of tube T2 and an increase in voltage drop through the cathode resistor I'I. Thus the potential of cathode I2 is made to increase, that is, to tend to follow the voltage im- The function of I6 so as to make it substantially equal to the bias on grid I5. l

Since the voltage applied to the grid I5 of the amplifier section of the double triode T2 is lowered as the result of the increase in signal input voltage to control grid I and since the potential f climmon cathode I2 of tube T2 is increased, the two changes produce a gross change in grid-to-cathode potential of the amplifier section that causes the plate current flowing through the load resistor I I to decrease, resulting in a decrease in the voltage drop in such load resistor. This change in voltage drop of the load resistor Il constituting the output may be fed into subsequent stages of the amplifier, representing the amplified signal of the input signal impressed on the control grid I of tube T1.

The amplifying system described also acts as a regulator to control variations .of cathode emission current, generally referred'\to as cathode drift. If it is assumed that for isome reason, other'than changes in the input current or voltage, the cathode emission of tube T1 is increased, the plate current and space charge grid current of T1 will both increase as a result of such increase in cathode emission. The increase in space charge grid current and plate current causes a decrease in the positive bias applied to both of the grids I5 and I6 of double triode tube T2. This decrease in positive potential on the grid I5 of the regulating half of tube T2 causes a decrease in the current iiowing through cathode resistor I1, resulting in a lowering of the cathode potential. The values of resistors 6, 8 and 9 are so chosen that for a given change in cathode emission current emitted by cathode 4 the change in voltage drop through resistor 5 will be equal to the change in voltage drop in-resistor 9 causing an equal change of potential on the cathode I2 and the grid I5. Since the potential of both the cathode I2 and the grid I5 of the amplifier section of tube T2 are changed in the same direction and by the same amount, there will be no net change in the relative potential therebetween. The plate current in the amplifying section of tube T2 therefore remains constant for a steady signal input. More particularly, the plate current flowing through load resistor I8 will be independent of variations in cathode emission current of tetrode tube T1 occurring otherwise than by changes in the input signal.

An outstanding advantage is obtained in the circuit illustrated by virtue of the fact that there is no metallic connection between space charge grid 3 and cathode I2, either directly or through resistors, the space charge grid 3 being connected solely to grid I6. By such connection solely to grid I5, changes in space charge grid current of tube T1 are greatly amplified by the regulator section of tube T2 thereby giving an amplified current flow and relatively high voltage drop across cathode resistor I'I thereby magnifying the voltage follow-up effect of cathode I2 relative to grid I6. If, on the other hand, space charge grid 3 were connected to cathode I2, either directly or through resistors, the space charge -grid circuit would have to furnish the entire current ow through the cathode resistors to cause the necessary voltage drop to make cathode I2 change in potential in accordance with changes in space charge grid current, hence comparatively small voltage changes of cathode I2 will take place since there is no magnification of the space charge grid current changes.

To summarize the amplifying and regulating action of the circuit described, it may be stated that a change in input voltage of tube T1 in one direction causes the grid and cathode of the control or regulator section of tube T2 to change in the same direction but causes the grid of the amplifier section of tube T2 to change in an opposite direction. thereby causing a net change in the relative potential of the grid and cathode in the ampliiler section, hence in the value of current now through the load resistor; on the other hand. `with constant input voltage, a variation t in a particular direction of the cathode emission i ci the tube T1 causes the grid and cathode of not only the control or regulator section. but of the amplifier section as well, oi tube T: to move in the same direction. thereby causing no net change in the relative potential oi' the lgrid and cathode in the amplifier section, hence causing no `change in current flow through the load resister. In other words, voltages of like polarity are applied to the second discharge device in such manner as to prevent translation therethrough of the eects of cathode drift of the first discharge device and the voltages of opposite polarity are applied to the second discharge device so as to convert said voltages into a fluctuating output voltage.

Thus it will be seen that I have provided an amplifying system suitable for amplifying direct currents or alternating currents, particularly those under 1,000 -cycles per second, that is highly useful to amplify very small currents, for example. currents under 104 amperes; that I have incorporated in such amplifying system a circuit for coupling a space discharge device to a second space discharge device comprising afurther amplifying stage and arranged so that changes in cathode emission current in the first space discharge device will not be amplified or translated through the second space discharge device or through subsequent stages of the amplii'ying system. I have thus provided a highly sensitive amplifier that incorporates features tending to make the output signal a true amplified replica oi the incoming or input signal, resulting in high precision measurement or translation of minute currents. Furthermore, I have provided a novel method of translating an amplified signal devoid of distortions caused by cathode drift.

The above-described amplifying system should be regarded merely as illustrative and not limiting insofar as the present invention is concerned, and the invention should be restricted only insofar as set forth in the following claims.

What is claimed is:

i. An electrical amplifier comprising, in combination, a discharge device including a control electrode having an input signal voltage impressed thereon, and a pair of electrodes whose voltages change in opposite directions for a given uni-directional change in input signal voltage and change in the same direction for a given uni-directional change in cathode emission current resulting from causes other than that produced by a change in signal input, circuit means including two triodes having grids connected, respectively, to said pair of electrodes and havingi interconnected cathodes, resistance means for making said triode grids vary in voltage the same amount and in the same direction for said given uni-directional change of cathode emission current of said first-mentioned discharge device, resistance means in series with one of said triodes to make its cathode follo-w the potential ci its grid and to make the grid and cathode of the other triode ineiective to translate said changes of cathode emission current occurring in said first discharge device, and an output circuit controlled solely by fluctuations in anode potential of said last-mentioned triode.

acuario 2. In a multi-stage `amplifier adapted for ampliilcation o! alternatingor direct current. an input stage comprising a space discharge device thathas a cathodaspace charge electrode, control eiectrode and an output electrode, an input circuit for said stage comprising said cathode and control electrode, an output circuit for said stage comprising saidspace charge electrode and said output electrode, circuit means including two triodes having grids connected, respectively, to said output electrode and said space charge electrode and having interconnected cathodes, resistance means for making said triode grids vary in voltage the same amount and in the same direction for a given uni-directional change of cathode emission current of said first-mentioned discharge device resulting from causes other than that produced by a change in signal input, resistance means in series with one of said triodes to make its cathode follow the potential of its grid and to make the grid and cathode of the other triode ineffective to translate said change of cathode emission current occurring in said first discharge device and an output circuit controlled solely by fluctuations in anode potential of said last-mentioned triode.

3. In a multi-stage ampliiier adaptedfor ampliiication of alternating or direct current, an input stage comprising a space discharge device that has a cathode, space charge electrode, control electrode and an output electrode, an input circuit for said stage comprising said cathode and control electrode, an output circuit for said stage comprising said space charge electrode and said output electrode, circuit means including a double triode, each section of which has a grid, said grids connected, respectively, to said output electrode` and said space charge electrode and having a common cathode disconnected from said space charge electrode, resistance means for making said triode grids vary in voltage the same amount and in the same direction for a given uni-directional change of cathode emission current of said first-mentioned discharge device resulting from causes other than that produced by a change in signal input, resistance means in series with one of said triodes to make said common cathode follow the potential of its grid and to make the triode including said common cathode and said grid connected to said output electrode ineffective to translate changes of cathode emission current occurring in said first discharge device, and an output circuit including said lastmentioned triode and controlled solely by anode potential changes thereof.

4. A direct current amplifier comprising, in combination, a discharge tube including a plate. cathode, control electrode and space charge electrode, an input circuit including said control electrode and cathode and an output circuit including said plate and space charge electrode, a source of direct current plate potential. a resistor connected between said plate and the positive terminal of said source, and a second resistor connected between said space charge electrode and said positive terminal of said source, said resistors being of such values that their voltage drop changes for given uni-directional changes in cathode emission current are substantially ing connected to the positive terminal of said plate potential source through a load resistor, and an output circuit connected to said lastmentioned triade anode and controlled solely by changes in voltage drop across said load resistor thereby translating an amplied signal devoid of cathode drift effects originating-in said firstmentioned discharge tube.

BARTON L. WELLER.

REFERENCES CITED The following references are of record in the file of this patent:

Number Number UNITED STATES PATENTS Name Date Warner July 21, 1925 Soller Dec. 6, 1938 FOREIGN PATENTS Country Date Germany Feb. 23, 1922 

